This invention relates to radio receiver, and more particularly to a radio receiver such as a car radio for receiving a radio wave from a broadcasting station and sensing a reception status of a radio wave from another broadcasting station during the reception of the first-mentioned radio wave.
Radio receivers of the type referred to equipped, for example, on a motor vehicle can normally receive a radio wave, such as an FM broadcasting wave, from a broadcasting station within a service area thereof but when the radio receivers go out of that service area, they can not well receive the radio wave. At that time, it is required to select a different broadcasting station having a good reception status in place of the broadcasting station received up to that time. Accordingly, each time the reception status becomes poor, the user must perform the operation of selecting a broadcasting station.
Now, in Europe, for example, there are regions each including a plurality of broadcasting stations disposed in scattered relationship therein and transmitting a common broadcast program at different frequencies. In those regions, it has been considered that the user receives a radio wave from a broadcasting station (which is called hereinafter "the present station") while he or she can sense the reception status of radio waves from a plurality of broadcasting stations transmitting the common broadcast program at different frequencies. Then, when the present station's reception status deteriorates he or she switches to that radio wave having a good reception status.
Thus, there have been proposed radio receivers having a pair of tuners disposed thereon, one of which is tuned to the radio wave from the present station and the other of which is tuned to that from a different broadcasting station for the purpose of sensing the reception status of the latter wave. However, those radio receivers have come into question in that they are expensive and large sized and that a single antenna is used with the pair of tuners to selectively receive a pair of radio waves, resulting in a decrease in the sensitivity of the antenna.
In order to solve this question, a method has been considered by which a radio receiver having a single tuner receives a radio wave from the present station and, by suspending the reception of the present station for an extremely short time interval, the receiver senses the reception status of a radio wave from a different broadcasting station within that time interval.
As an example, a radio receiver forming the prior art of the present invention has comprised a usual superhetrodyne receiver unit including a muting circuit, a digital frequency synthesizer in the form of a phase locked loop including a voltage controlled oscillator connected to a frequency mixer included in the superhetrodyne receiver unit and having its output fedback thereto through a prescaler, a programmable divider, a phase detector applied with a reference frequency and a loop filter.
In operation, a control unit applies first a frequency division ratio for the present station to the programmable divider to lock the phase locked loop to the frequency of the present station. At that time, the radio receiver receives a radio wave from the present station by having the voltage controlled oscillator generating a local oscillation frequency required for doing so. Then, the resulting reception level is sensed and stored.
If an attempt is made to sense the reception status of a different broadcasting station during the reception of the present station, then the control unit applies a muting pulse with an extremely short duration to the muting circuit to mute an output therefrom while at the same time the control unit applies a loop band changing pulse to the loop filter to decrease the time constant thereof so as to thereby accelerate the locking of the phase locked loop. Simultaneously, the control unit applies a frequency division ratio for the different broadcasting station to tune the receiver to that station as described above. The resulting reception level is sensed and compared with the stored reception level. When the present station is determined to be higher in reception level than the different station, the control unit applies the frequency division ratio for the present station to the programmable divider to permit the receiver to receive the present station.
The change of the present to the different station and the return of the different to the present station should be accomplished within the extremely short duration of the muting pulse which has been experimentally found to be of 5 milliseconds or less with the satisfactory result. Otherwise, the human auditory feeling is impeded.
In order to decrease the time interval for which the reception of the present station is suspended, it is necessary to decrease the frequency divisor or the reciprocal of frequency division ratio of the prescaler and that of the programmable divider and to increase the reference frequency as a result of a mathematical analysis of the phase locked loop.
The majority of integrated circuit elements employed with conventional phase locked loops for the FM band have the frequency divisor of their prescaler set to be not less than ten (10) and have their reference frequency set to be not higher than 5 kilohertz in view of their manufacturing processes. Therefore, conventional phase locked loops have been disadvantageous in that the response time thereof is increased so as to lengthen the time interval for which the reception of the present station is suspended during the reception thereof.
Also, when the phase locked loop has a high natural angular frequency the pulling-in time thereof is decreased, resulting in an increase in jitter of the voltage controlled oscillator which acts as a local oscillator. Therefore, the receiver has a decreased signal-to-noise ratio.
Accordingly, it is an object of the present invention to provide a new and improved radio receiver including a phase locked loop having an increased response speed and having an extremely fast changeover speed when locked to the frequency of a broadcasting station which is changed from the present station to another station following the suspension of the reception of the present station.